The present invention relates to rotating machinery and, more particularly, to large DC rotating machinery.
DC motors and generators conventionally include stationary field windings on a stator surrounding armature windings rotatably disposed on a rotor. A commutator affixed to the rotor has a plurality of copper bars arranged into a cylinder to provide electrical connection to individual ones of the armature windings on the rotor. A plurality of stationary carbon brushes make sliding contact with the commutator for connecting power to (in the case of a motor) or connecting power from (in the case of a generator) the armature windings. The brushes and commutator effectively switch the power connections to the armature windings in a manner effective to maintain the magnetic poles of the armature at an electrical angle of about 90 degrees to the magnetic poles of the field windings. The resulting quadrature of the two magnetic fields produces the power when the machine is driven as a generator and produces the torque when the machine is run as a motor.
One of the most important limitations on the capability of a DC machine is the ability to transfer the necessary armature current through the brush-commutator interface without destructive sparking which may result in rapid loss of both brush and commutator material. The physical processes taking place at the interface between a copper commutator bar moving with respect to a carbon brush, although they are very old and almost universally used in DC machinery, are not well understood. The brushes are positioned so that they pass from one commutator bar to the next at the time that the associated armature windings are located at the midpoint between adjacent magnetic poles of the stator. If the stator were the only source of magnetic flux on the armature, this positioning should permit switching to take place while the coils being switched are at zero volts. However, other sources of magnetic flux are present. The principal one for present purposes is the magnetic flux due to the current in the armature itself. The armature flux, being physically in quadrature with the field flux, adds to the field flux in some locations and subtracts in other. Because of nonlinear saturation of the iron, the additive and subtractive flux contributions are unequal and, as a consequence, the net magnetic flux in the air gap midway between magnetic poles of the field does not go to zero when an armature winding moves past this location but instead a substantial flux on one polarity is present in the air gap midway between alternate pairs of field poles and a flux of the other polarity is present between the remaining pairs of poles. This effect produces a substantial voltage between the commutator bars and the brushes during commutation and leads to sparking. Such sparking is acceptable in small capacity DC machines but is fatal to brushes and commutators in larger machined.
In larger DC machines, the flux contribution produced by current in the armature windings during the short period while the brushes are passing from one commutator bar to another is cancelled by a commutating or interpole winding located midway between adjacent field poles. The commutating winding is cnnected in series with the armature current so that the cancellation is correct at all loads and speeds.
For large DC machines subjected to heavy overloads, rapidly changing loads or operation with a weak field, a further phenomenon can lead to commutator flashover. The brush-copper interface produces a surface condition which is similar to a gas plasma. This surface condition is favorable to breakdown between adjacent commutator bars under an applied voltage. Consequently, a maximum allowable voltage between adjacent commutator bars is on the order of 30 or 40 volts. Under transient conditions of changing load, or under high steady load, the distortion of the magnetic flux distribution in the air gap between the field and armature windings contributed by armature current may permit voltages to be induced in the armature windings connected to adjacent commutator bars that are sufficiently high to initiate arcing or flashing between the commutator bars. Flashing between commutator bars may quickly spread around the entire commutator and, in addition to the destructive effect this may have on the commutator, it also acts as a dead short across the line.
To counter the flashover problem, pole face windings are provided in the face of the stator poles. These pole face windings consist to conductive bars disposed in slots in the iron of the stator poles and connected in series with the armature current. The axis of the magnetic flux produced by the pole face windings is of opposite polarity and aligned with the axis of the magnetic flux of the armature. Thus, properly sized and positioned pole face windings are capable of cancelling the flux contibution of the armature current and thus provide relatively close control of the magnetic flux in the entire air gap between the field windings and the armature. This permits reducing the flux distortion in the air gap to a low enough value to reduce or eliminate flashing even under severe operating conditions favorable to its initiation.
As noted above, pole face windings are conductive bars disposed in slots in the field poles. Since the bars of the pole face windings must be connected in series with the armature current, these bars are maintained at the applied line voltage. The field poles are customarily grounded. Thus insulation is required between the bars and the iron on the field poles. Such insulation is provided by an insulating layer about the bars. The insulating layer is protected from damage by a hard, damage-resistant slot armor between it and the iron of the field pole. Such insulation and slot armor is disclosed in U.S. Pat. Nos. 3,454,804 and 3,801,392. The '804 patent also discloses an insulation on the end turns external to the slot.
In more modern practice, the insulating layer is a multiple-layer wrapping of a high-temperature non-woven polyamide paper such as, for example, Nomex manufactured by the DuPont company. The polyamide paper layer extends beyond the slot at both ends of the field pole. A wrapping of a glass fabric tape impregnated with partially cured resin over the polyamide paper is shaped and heat cured to provide a damage-resistant armor. The armor layer extends beyond the polyamide paper at both ends of the bar and is conformed as closely as possible to the surface of the bar in order to reduce the entry of comtaminants, and particularly conductive contaminants, therebetween.
It is desirable to achieve as low an electrical leakage current as possible between the filed iron and the pole face bars. This is sometimes difficult to achieve since the operating environment often contains conductive particles such as, for example, carbon dust from the brushes or a conductive ore, which can adhere to the surface of the slot armor and reduce the resistance between field pole and pole face bars to as low as, for example, 25 to 50 Kilohms. Many large DC machines include trip circuits which are occasionally actuated by leakage resistance of this magnitude to produce nuisance trips which thereupon require analysis and resetting. Lower electrical leakage current is achieved when as long an electrical leakage current path as possible is provided between the pole face bars and the iron of the field.
The length of the electrical leakage path is reduced significantly if conductive materials find their way into a space between the armor and the pole face bar. Unfortunately, conventional armor materials do not adhere well to the copper metal of which pole face bars are conventionally made. Furthermore, once cured, the armor becomes a rigid body having a temperature coefficient of expansion which is substantially different from the copper pole face bars. This, of course, encourages the formation of a gap between the armor and the copper into which conductive contaminants can migrate. When this problem is combined with the formation of a conductive contaminant coating on the outer surface of the armor, relatively short electrical leakage current paths rapidly develop. Combatting the resulting low leakage resistance has required substantial maintenance labor to clean the conductive contaminant layer from the outer surface of the armor. Even with the expenditure of substantial maintenence labor, this technique is sometimes only marginally successful.
In a further attempt to achieve long electrical leakage current paths it has become common to apply a uniform coating of paint such as, for example, polyurethane enamel paint, using, for example, electrostatic painting techniques to the entire surface of the field pole iron, the surfaces of the armor extending therebeyond and the surfaces of the copper bars extending beyond the armor. The paint layer may also be applied to the surfaces of the end turns which are used to interconnect the ends of the pole face bars. Unfortunately, the paint layer has been found to develop a crack at the end of the armor. Such a crack permits the migration of conductive contaminants therethrough and into the gap that forms between the pole face bar and the armor. This substantially subverts the attempt to increase the electrical leakage current path length by the addition of the paint layer since the electrical leakage current path from the field pole iron to the pole face bar begins at the end of the armor as it did without the paint coating.
A heat-shrinkable Mylar tube was used for bridging the end of the armor to the surface of the pole face bars. It was found that, not only does paint not adhere well to the surface of Mylar, but also conductive contaminants were still able to migrate under the Mylar tube. Furhtermore, even surface cleaning of the Mylar was unsuccessful in stopping the leakage.